In the 'K-index versus time'-graph
(Fig 1), visualizing the disturbances of the earth
magnetic field in Wingst (Germany), you can see the recursive pattern of a coronal hole (indicated by the
two green circles). A period of 27 days
is approximately with the time the Sun needs to turn once around her axis.
Checking the graphs of SOHO/CELIAS and ACE , we see that the peaks in
the density graph (CELIAS, Fig 3) and a more negative B_z component of the
interplanetary magnetic field (ACE, Fig 4) coincide with the 2 periods of
geomagnetic disturbances. The second period was less fierce in terms
of the length in time as the solar input was
less strong: a smaller density peak and a less negative B_z component
compared with the first period. However, the k-index in Wingst reached the value
of 6 compared with a 5 in the first period. The estimated K_p index from
NOAA (Fig 2 ) however behaved differently. The global index reached only a 5 during
the second period compared with a 6 in the first period.